Protein phosphorylation/dephosphorylation plays a central role in the regulation of a variety of cell functions, such as cell proliferation, differentiation, and signaling processes. Uncontrolled signaling has been implicated in a variety of disease conditions including inflammation, cancer, arteriosclerosis, and psoriasis. It is estimated that more than 1,000 of the 10,000 proteins active in a typical mammalian cell are phosphorylated. As is well-known in the art, high energy phosphate, which drives activation, is generally transferred from adenosine triphosphate molecules (ATP) to a particular protein by protein kinases and removed from that protein by protein phosphatases.
The presence or absence of a phosphate moiety modulates protein function in multiple ways. A common mechanism involves changes in the catalytic properties (Vmax and Km) of an enzyme, leading to its activation or inactivation.
A second widely recognized mechanism involves promoting protein-protein interactions. An example of this is the tyrosine autophosphorylation of the ligand-activated EGF receptor tyrosine phosphatase. This event triggers the high-affinity binding to the phosphotyrosine residue on the receptor's C-terminal intracellular domain to the SH2 motif of an adaptor molecule Grb2. Grb2, in turn, binds through its SH3 motif to a second adaptor molecule, such as SHC. The formation of this ternary complex activates the signaling events that are responsible for the biological effects of EGF. Serine and threonine phosphorylation events also have been recently recognized to exert their biological function through protein-protein interaction events that are mediated by the high-affinity binding of phosphoserine and phosphothreonine to WW motifs present in a large variety of proteins.
A third important outcome of protein phosphorylation is changes in the subcellular localization of the substrate. As an example, nuclear import and export events in a large diversity of proteins are regulated by protein phosphorylation.
Reversible protein phosphorylation is an essential regulatory mechanism in many cellular processes. While the post-translational modification alters the properties of key regulatory proteins involved in various biochemical pathways, protein kinases and phosphatases themselves are subject to control through the action of extracellular signals such as hormones and growth factors. Although much attention has been paid to the regulation of protein kinases, it is now apparent that protein phosphatases are also highly regulated enzymes that play an equally important role in the control of protein phosphorylation.
Protein phosphatases may be roughly divided into three families based on their substrate: the serine/threonine (S/T) phosphatases, the tyrosine phosphatases, (PTP), and dual specificity phosphatases.
Serine/threonine (S/T) protein phosphatases (PPases) catalyse the dephosphorylation of phosphoserine and phosphothreonine residues. Their action is opposed to that of a large number of serine/threonine protein kinases. In mammalian tissues four different types of PPase have been identified and are known as PP1, PP2A, PP2B and PP2C. Except for PP2C, these enzymes are evolutionary related. The catalytic regions of the proteins are well conserved and have a slow mutation rate, suggesting that major changes in these regions are highly detrimental.
Protein tyrosine phosphatases (PTPs) catalyse the dephosphorylation of phosphotyrosine residues. PTPs represent a large family of enzymes that play a very important role in cellular signaling within and between cells. PTPs work antagonistically with protein tyrosine kinases (PTKs) to regulate signal transduction in a cell.
A few protein phosphatases have dual specificity and dephosphorylate serine/threonine and tyrosine residues. This family now includes major regulators of growth cycle such as p80cdc25, as well as phosphatases that regulate the mitogen-activated protein kinase pathway.